Polyfill Node.is_part_of_edited_scene()

Godot 4.3+ exposes `Node.is_part_of_edited_scene()` to GDScript, which is
analogous to `Engine.is_editor_hint()` but determines whether the `@tool`
script's node is itself being edited, as opposed to instantiated elsewhere in
the editor.

Reimplement this function in GDScript.

https://phabricator.endlessm.com/T35540

Move node_is_part_of_edited_scene to new Util module

This will allow it to be reused in other scenes which have the same problem as
tooltip did.

Author: wjt

addons/block_code/ui/util.gd

@@ -0,0 +1,6 @@
+extends Object
+
+
+## Polyfill of Node.is_part_of_edited_scene(), available to GDScript in Godot 4.3+.
+static func node_is_part_of_edited_scene(node: Node) -> bool:
+	return Engine.is_editor_hint() && node.is_inside_tree() && node.get_tree().edited_scene_root && node.get_tree().edited_scene_root.get_parent().is_ancestor_of(node)